PARENT SESSION
Oral Session # 67: Urban Ecology I: Dynamics, Values, and Systems.
Presiding: A Yeakley
Thursday, August 7. 8:00 AM to 11:30 AM, SITCC Meeting Room 105.

Discontinuities in urban systems over time.

Garmestani, Ahjond^{*,1, 2}, Allen, Craig^{1, 2}, Bessey, K. Michael², ¹ South Carolina Cooperative Fish and Wildlife Research Unit, Clemson, SC, USA² Clemson University, Clemson, SC, USA

ABSTRACT- Complex systems, such as ecosystems and urban systems, emerge unpredictably without the influence of central control but as a result of adaptive behavior by their component, interacting agents. Urban systems exhibit spatial patchiness in their social and economic infrastructure. Thus, urban systems, much like ecosystems, are subject to a hierarchy of structure and processes that govern the function and growth of cities at a variety of scales. Urban size distributions have been described by Zipf′s law or the rank-size rule, which imply invariant growth processes across scales. Zipf′s law predicts that city-size distributions will have a continuous distribution, and conform to the restraints of a power law. We analyzed city size distributions, by decade, from the southwestern region of the United States for the years 1890-1990. We determined if the distributions were discontinuous and changes in the pattern of discontinuities over time. Discontinuities were determined with computer simulations utilizing a null model, cluster analysis and SMW analysis. The data were discontinuous as determined by all 3 methods, with agreement among all comparisons. In 1890, the southwestern United States region consisted of 48 cities, but by 1990 this had grown to 161 cities. Likewise, the maximum city size increased from 38,067, to 3,198,259 during the same time period. Despite these enormous changes, our analyses identified 5 or 6 aggregations and discontinuities in each of the decades analyzed. Our analyses suggest that the spatial and temporal processes shaping urban size distributions at different scales are conserved over time despite great change in city function and size.

Key words: discontinuity, complex systems, size distribution, urban system